This invention relates to an economical continuous process for the preparation of quinacridone pigments having uniform particles of a narrow particle size distribution.
Processes for the preparation of quinacridone are known. E.g., S. S. Labana and L. L. Labana, "Quinacridones" in Chemical Review, 67, 1-18 (1967), and U.S. Pat. Nos. 3,157,659, 3,256,285, and 3,317,539. The quinacridones thus obtained, known as crude quinacridones, are generally unsuitable for use as pigments and must undergo one or more additional finishing steps to modify the particle size, particle shape, or crystal structure to achieve pigmentary quality.
A preferred method for preparing quinacridones involves thermally inducing ring closure of 2,5-dianilinoterephthalic acid intermediates, as well as known aniline-substituted derivative hereof, in the presence of polyphosphoric acid (e.g., U.S. Pat. No. 3,257,405) or even sulfuric acid (e.g., U.S. Pat. No. 3,200,122 and European Patent Application 863,186). After ring closure is complete, the melt is drowned by pouring into a liquid in which the quinacridone is substantially insoluble, usually water and/or an alcohol. The resultant crystalline pigment is then further conditioned by solvent treatment or milling in combination with solvent treatment.
Final particle size of quinacridone pigments can be controlled by the methods used in both synthesis and aftertreatment. For example, quinacridone pigments can be made more transparent by reducing the particle size or more opaque by increasing the particle size. In known methods, particle size is generally controlled during precipitation of the pigment by drowning or during milling or solvent treatment of the crude pigment. Tinctorial strength and transparency of pigments can also be affected by solvent treatment. Aftertreatment steps that manipulate the crude pigments particle size are often referred to as conditioning methods.
Although batchwise methods can produce good quality product, a more efficient continuous process would be desirable. Continuous process methods have been reported for other types of pigments, particularly copper phthalocyanines and perylenes (see U.S. Pat. Nos. 2,964,532, 3,188,318, and 5,247,088), but until now have not been reported for the preparation of quinacridones. The present invention provides such a continuous process for preparing and drowning quinacridones using smaller amounts of dehydrating agent than used in standard methods, even when such smaller quantities of the dehydrating agents produce high viscosities. In addition to allowing the use of smaller quantities of dehydrating agent, which would lower manufacturing costs and reduce environmental impact, the present invention produces quinacridones having a desirably narrow particle size distribution. The pigment particles are also generally smaller than those produced in batch processes and can be conditioned to produce deeper, more transparent pigments.